This application addresses the NORA priorities Indoor Environment and Intervention Effectiveness Research. We propose to conduct a blinded experimental intervention study of the independent effects of outdoor air supply rates and active building humidification on absence rates, incidence of respiratory infections characterized by RT-PCR, and non- specific building-related symptoms among office workers at Polaroid. We will assign 20 office areas to altered use of humidification equipment and ventilation in a random block design. Each area will be studied for 1 year with four 3-month intervention cycles. Start and end dates will be staggered so that the entire study extends over 25 months. Thus, each intervention will be tested in multiple groups of workers in every season over 2 years. We will use as outcomes absence from work, incident respiratory infections characterized by RT-PCR of nasal lavage, and nonspecific building-related symptoms. We are motivated by our earlier finding that illness absence rates in Polaroid offices with lower ventilation rates (25cfm/person) were 54 percent greater (p less than 0.001) than in offices with higher ventilation rates (50cfm/p) independent of complaints. Active humidification was also associated with increased absence rates. We will test the following hypotheses: 1) Low outdoor air supply rates, and active humidification will increase sick leave among office workers in air-conditioned buildings. 2) Low outdoor air supply and active humidification will increase the frequency of symptomatic respiratory infections and the frequency with which office mates are infected by the same organism - accounting for the impact of these factors on absence rates. 3) The effect of ventilation and active humidification on respiratory infections and absence is independent of perceived indoor environmental quality and non-specific building related symptoms. 4) The cost of the experimental interventions is less than the cost of lost productivity due to sick leave. In this study we bring to bear the use of objective health outcomes and molecular biology to identify the broader dimension of the impact of buildings on worker health and productivity. This study will provide objective data, on the health impact of the modern office environment. By defining the role of airborne infection in these health effects, this study will lead to new and more cost-effective approaches to protecting the health of workers.